The present invention relates to a driving system for a thermal recording head as used in facsimile devices and so forth.
Particularly in digital facsimile, since the compressibility of information for transmission varies according to the original (manuscript), the driving period is not fundamentally constant at the recording means. The situation is as illustrated in FIG. 1. FIG. 1(A) represents the arrangement in time of each line information Vi (i= 1, 2, 3, . . .) in transmission. Information V.sub.1 for the first line is transmitted at time t.sub.1, information V.sub.2 for the second line is transmitted at time V.sub.2, and that for the n-th line is transmitted at time t.sub.n. A receiver decodes and records each line signal P.sub.1, P.sub.2, . . . in constant time intervals t.sub.0 as shown in FIG. 1(B). Thus, there arises an idle time between each line recording, for example up to the line signal P.sub.2 after P.sub.1 is recorded, and the idle time is not constant.
In a thermal recording where the idle time varies between each line recording as just mentioned, an unevenness in recording density follows, much as in other recording systems. The reason is that waste heat from the previous line recording is subjected to the influence of the idle time between each line recording, and the change in waste heat may exert an influence on the recording of the next line.
A conventional method of heating control comprises detecting the temperature of a head support on which a heating resistor is arranged, i.e. a dielectric substrate with a temperature detecting element like a thermistor, and controlling the energy to be impressed on the heating unit according to the detected temperature. However, this method is not satisfactory for improving the unevenness in density according to the change in waste heat. The reason is that it is difficult with respect to time and also with respect to temperature change, to detect the influence of the waste heat from the previous line recording operation according to the detected temperature of the dielectric substrate.